CUSTOMER FUNDS - Assets Underlying Customer Funds (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Customer funds	$ 2,976,428	$ 2,037,832
Customer Funds In Transit
Debt Securities, Available-for-sale [Line Items]
Customer funds	0	262,562
U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Customer funds	39,521	113,178
U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Customer funds	338,455	333,346
Cash
Debt Securities, Available-for-sale [Line Items]
Customer funds	20,584	145,577
Cash Equivalents | Money market funds
Debt Securities, Available-for-sale [Line Items]
Customer funds	1,655,962	777,193
Cash Equivalents | Reverse repurchase agreement
Debt Securities, Available-for-sale [Line Items]
Customer funds	921,906	246,880
Cash Equivalents | U.S. agency securities
Debt Securities, Available-for-sale [Line Items]
Customer funds	0	47,300
Cash Equivalents | U.S. government securities
Debt Securities, Available-for-sale [Line Items]
Customer funds	$ 0	$ 111,796